This invention relates generally to flowmeters of the variable-area tube type, and more particularly to a protective shield and bracket assembly for such flowmeters.
Flowmeters of the variable-area tube type are usually designated "rotameters," the term being derived from the fact that in early versions of such meters, the weighted plummet or float movable in the upright tube had slots therein to impart a rotational force thereto for the purpose of stabilizing and centering the float. The present trend, however, is toward non-rotating floats.
In a rotameter, the fluid to be metered, which may be a gas or liquid, is admitted to the lower end of the tapered bore in the tube and emerges from the upper end thereof, the float being raised to a position of equilibrium between the downward gravitational force of the float and the upward force of the fluid flowing through the annular orifice surrounding the float. The float assumes a vertical position that depends on the rate of fluid flow. This position is indicated along a calibrated scale on the front of the tube. Because the meter tube is usually made of thin-walled glass and is relatively fragile, in existing types of variable-area tube flowmeters, the tube is supported by end fittings within a meter body adapted to protect the tube against stresses as well as to effect tight sealing thereof.
The pressure rating of a variable-area metering tube depends on the tensile characteristics of the glass as well as on its wall thickness. However, the maximum pressure rating of a given meter affords no assurance that breakage of the tube will not occur at pressures below its maximum rating.
Inherent material limitations can result in tube breakage. Glass is a brittle material which may break upon accidental impact. Breakage may also occur if the glass is subjected to thermal shock. Moreover, an exposed glass tube of good inherent strength may be weakened by surface scratches, nicks or cracks therein that may cause breakage when the tube is pressurized or subjected to vibrations. Incorrect installation or faulty operating methods can also give rise to tube breakage regardless of operating pressure. And with certain highly reactive fluids, the glass may erode evenly so that wear is not noticeable and there is no warning that the tube requires replacement to avoid breakage.
Should a variable area flow tube shatter in the course of operation, injuries may be inflicted on personnel in the vicinity thereof. Because the glass tube is under pressure, the hazards created by the resultant explosion is not only from the fragments of a glass but also from the suddenly released fluid which may be at high temperatures or contain dangerous chemicals.